The present invention relates to resonant microbeam sensors that utilize induced strain to measure acceleration, pressure and other variables, and more particularly to providing temperature compensation in such transducers.
Resonant transducers are well known for their capacity to achieve high accuracy measurements. Vibrating transducers have been used in precision accelerometers and pressure sensors. These devices operate on the principal that a natural frequency of vibration (i.e. resonant frequency of an oscillating beam or other member) is a function of the induced strain along the member. More particularly, tensile forces elongating the beam increase its resonant frequency, while forces compressing the beam reduce the natural frequency. The frequency output of a resonant gauge or beam is readily converted to digital readings reflecting the measured quantity, requiring only a counter and a reference clock for this purpose. Thus, such transducers are simple and reliable, provide a high degree of discrimination while using a relatively simple interface to digital signal processing circuitry.
One particularly effective transducer of this type is a resonant integrated microbeam sensor, for example as disclosed in U.S. patent application Ser. No. 07/937,068, filed Aug. 31, 1992 entitled "RESONANT GAUGE FOR MICROBEAM DRIVEN IN CONSTANT ELECTRIC FIELD" and assigned to the assignee of this application. The sensor includes a silicon substrate, a polysilicon flexure beam attached at both ends to the substrate, and a polysilicon rigid cover cooperating with the substrate to enclose the flexure beam within a sealed vacuum chamber. A pair of bias electrodes on opposite sides of the beam create a constant electrical field about the flexure beam. A drive electrode on the flexure beam is selectively charged to oscillate the beam. A piezo resistive element on the flexure beam is used to indicate beam position, and also provides feedback to the drive oscillator. Thus, the beam tends to oscillate at its natural resonant frequency.
The sensor can be fabricated on a pressure sensitive diaphragm to be elongated or compressed by deflections of the diaphragm in response to pressure changes. Similarly, the sensor can be fabricated on a flexure of an accelerometer to be elongated or compressed by deflections of the flexure in response to accelerations. While satisfactory in many of these applications, the sensors are subject to error due to deviations in temperature.
It is known to provide temperature compensation in connection with resonant sensors. For example, U.S. Pat. No. 4,535,638 (EerNisse et al) discloses a resonator transducer system in which a vibratory element such as quartz crystal is driven to oscillate at two frequencies, both of which vary with changes in applied force and changes in temperature. The frequency outputs are processed by a computer containing predetermined coefficients for correcting as to the temperature effect.
U.S. Pat. No. 4,765,188 (Krechmery et al) discloses a pressure transducer including a diaphragm with several piezo resistor strain gauges for sensing pressure. A temperature dependent resistor also is formed on the diaphragm. The output of the temperature sensitive resistor is converted to a digital signal provided to a programmable read-only memory (PROM). The PROM stores correction data to provide temperature compensation.
While these approaches are workable, they require storage of compensation data and also frequently require analog-to-digital conversion. This adds to the complexity of sensing and compensation circuitry and thus increases the difficulty of semiconductor device fabrication.
Therefore, it is an object of the present invention to provide a resonant sensing device in which one or more sensors that sense temperature provided for compensation generate digital outputs, eliminating the need for analog-to-digital conversion.
Another object is to provide, in a single measuring device, the combination of a primary resonant sensor and at least one secondary resonant sensor for compensation.
A further object is to provide a measuring device in which a secondary resonant sensor effectively compensates for temperature, even when mounted proximate a primary resonant sensor on the measuring device.
Yet another object of the invention is to provide a temperature compensating resonant sensor that can be formed directly upon a pressure measuring diaphragm or an acceleration measuring flexure, and in each case be virtually unaffected by fluctuations of the diaphragm or flexure due to pressure differentials and strains, respectively.